Grabby rear brakes

[ghunt81]

Have any of you guys had an issue with the rear drums being grabby when it's wet outside, until they get warmed up? My truck is doing that, I had the same issue on my old '90 Mustang but I could never figure out a solution to it. Does anyone know what causes it and if there's a fix?

 

[RonD]

You should have rear ABS(anti-lock brake system), if not 4 wheel ABS, on a 2009 so shouldn't be grabby.

Usually cause, if it is just one wheel, is a slave that is sticking, or the adjuster isn't working on drum brake setup.
Brake line to a wheel can also be slightly plugged, when fluid is pushed thru it builds up pressure then releases suddenly expanding slave too much cause lock up.

Drum brakes have an internal star wheel between the lower part of the 2 shoes, slave is at the top.
When you back up and apply the brakes a lever pushes down on the star wheel and if there is space between the shoe and the drum the star wheel will turn pushing the 2 shoes apart at the bottom and closer to drum.
Same thing happens when you apply the emergency brake.
This keeps the shoes at the same distance from the drum as they wear down.

It doesn't adjust in forward driving.

If you park on the street and have an automatic transmission you may not back up enough or use the e-brake enough to keep the rear brakes adjusted.
You can simply pull out the e-brake release and pump the e-brake pedal to adjust the rear brakes, you can't over tighten them, the lever will slide over the points on the star wheel if it is tight already.
If they are out of adjustment you will feel the e-brake pedal resistance coming up higher and higher as the gap closes.

Pickup trucks with no weight in the bed can get rear wheel lock up which is why rear ABS was added years ago.
In the rain roads are wet so less traction and more likely to get wheel lock up, but no it shouldn't happen unless something is out of wack.

There is also a proportioning valve in the brake system that sends 70% of the brake pedal pressure to the front wheels and 30% to rear wheels in pickups, in cars it is 60/40 but this changes depending on model and year.
If there were problems here or with front brakes you could end up with equal or higher pressure at the rear, so rear lock up would happen since front brakes wouldn't be helping as much to slow vehicle down.

 

[Hagan]

I had the same problem on a '99 Dodge Cummins I owned. First thing in the morning when I'd come to a stop the rear tires would skid for a second and after that it was fine. I replaced the rear shoes and all was fine. Good luck!

 

[4.0blues]

I know exactly what you're talking about. When the truck's rear drums and shoes are cold from sitting overnight, and the air is moist, first few times of braking in the morning locks them up. After braking a couple of times, everything is fine. Really irritating.

A couple different mechanics I know told me it sounds like possible glazing on the shoes. They recommended new shoes and turning the drums. Between the pads and drum turning (plus time in my garage), didn't cost too much. They've been working great for some time now.

 

[gw33gp]

I think this phenomenon is due to the drum surface flash rusting overnight when it is damp. I get this same thing when it rains. During the first stop, the rear tires want to lock up even under light braking. Once this flash rust has worn off during the first or second braking, it works normal again.

I think the front brakes are effected also but not as much as the rear. I have noticed after sitting for an extended period in the rain, the brakes overall are very sensitive to light braking initially. Again, it goes back to normal after applying the brakes a time or two.